Duff played four full seasons and parts of two other seasons with the Canadiens, but made an impact immediately. Duff was seen as an offensive player, and twice hit the 20-goal plateau with the Habs.

In two of those years, he made the All-Star (plus another five times with other teams).

In his final playoffs appearance, Duff scored six goals and 14 points in 14 games with the Canadiens, helping them win the cup in 1969. He was traded the next season, and played for the Los Angeles Kings and the Buffalo Sabres, but never made the playoffs again.